Energy storage devices such as batteries or capacitors are being required to demonstrate higher functionality with respect to high energy density, high reliability and the like accompanying the reduced size and higher efficiency of electronic equipment in recent years.
An example of energy storage devices that have attracted particular attention are lithium ion secondary batteries. Lithium ion secondary batteries have characteristics such as nigh voltage, high energy density, long life and rapid charging speed. Lithium ion secondary batteries are already currently used in compact electronic devices such as cell phones, smartphones or laptop computers, and are expected to be used in large electronic equipment focusing primarily on vehicle-mounted applications such as electric vehicles or hybrid vehicles in the future.
Lithium ion secondary batteries are typically composed of a positive electrode active material, negative electrode active material, electrolytic solution (electrolytic solution/electrolyte/additive) and separator.
The role of the separator can be broadly classified into two roles. The first is to prevent the two electrodes having different charged states from physically making direct contact, (short-circuiting). As a result, a high level of safety can be realized and high reliability can be imparted to the finished product. In order to accomplish this, required properties of the separator consist of having adequate mechanical strength and not causing short-circuits during voltage loading. In order to prevent the occurrence of short-circuiting, the separator is required to have a fine fibril structure or uniform network structure. The second role of the separator is to ensure ionic conductivity between the positive electrode and negative electrode while retaining the electrolytic solution. With respect to this role, the separator is required to have high ionic conductivity in order to realise high capacity and high output. Other separator required properties consist of reduced overall thickness and the presence of numerous void portions inside the separator.
Examples of separators currently being used include porous films and nonwoven fabrics. These materials have the aforementioned properties of, for example, high mechanical strength, fine network structure, suitable thickness and numerous void portions. Consequently, they allow the production of highly reliable, high-performance lithium ion batteries.
A polyolefin-based porous film is an example of a porous film separator that is widely used for the separators of lithium ion secondary batteries. The porosity thereof is comparatively high and it is able to realise high rate characteristics. In addition, porous film separators also demonstrate a so-called shutdown effect, whereby safety is maintained by sacrificing battery performance by enabling microvoids to melt and close up when an excessively large current flows at a high temperature of 130° C. to 180° C., thereby enabling them to be used as separators for lithium ion secondary batteries.
On the other hand, separators composed of nonwoven fabric demonstrate high electrolyte retention, high battery rate characteristics and superior voltage retention rate due to their high porosity. In addition, they also offer the advantages of light weight and compatibility with large-volume production. They can also be expected to demonstrate high resistance effects by forming a resin having superior heat resistance. Moreover, since pore diameter can be controlled by controlling fiber diameter, numerous studies have been conducted on separators composed of nonwoven fabric
Patent Document 1 discloses an attempt to use a wet nonwoven fabric, in which heat-resistant fibers composed of a resin having a melting point or carbonization temperature of 300° C. or higher are immobilized by a thermoplastic resin, as a separator.
Patent Document 2 discloses an attempt to use a polyolefin-based fiber, which has a laminated three-layer structure consisting of an intermediate layer in the form of a nonwoven. fabric layer composed of micro fibers having a basis weight of 20 g/m2 or more and average fiber diameter of 5 μm or more, and upper and lower layers in the form of nonwoven fabric layers having an average fiber diameter of 5 μm to 20 μm, and is subjected to hydrophilic treatment, as a separator.
Uniformity of the thickness of a nonwoven fabric is important for ensuring uniformity of the chemical reaction when using the nonwoven fabric as a separator. In Patent Document 2, uniformity is realized by providing a melt-blown nonwoven fabric having high film uniformity for the intermediate layer of a nonwoven fabric typically having low film uniformity in the form of a spunbonded layer. However, since the uniformity of the spunbonded nonwoven fabric remains low, there are concerns over decreases in electrolyte mobility and properties of the electrolytic solution.
In Patent Document 3, an attempt is made to improve electrolytic solution retention by using a laminated nonwoven fabric obtained by laminating a melt-blown nonwoven fabric having an average fiber diameter of 0.5 μm to 3 μm and smoothing the surface thereof.
In addition, studies have also been conducted on methods for further improving separator performance. For example, Patent Document 4 discloses a separator in which insulating particles are coated on a porous base material as a technology for preventing short-circuiting caused by thermal contraction of the separator.